The present invention relates to an assembled camshaft. More specifically, the present invention pertains to a three-dimensional camshaft having a three-dimensional cam and its manufacturing method. The cam profile of the three-dimensional cam varies axially.
As generally known, for example, in a valve train of an on-vehicle internal combustion engine, an intake or an exhaust valve is selectively opened and closed by the rotation of a camshaft driven by an output shaft, or crankshaft. In recent years, a so-called three-dimensional camshaft has been proposed. The three-dimensional camshaft has a three-dimensional cam. The radius of the cam face changes in the axial direction of a camshaft, so that performance characteristics such as engine power and fuel consumption rate are optimized in accordance with engine operation conditions (Refer to Japanese Unexamined Patent Publication No. 3-179116). The camshaft varies valve characteristics such as intake valve opening time and exhaust valve closing time.
To change the valve characteristics, the camshaft is hydraulically moved in the axial direction. This changes the cam profile at the position where a follower, or valve lifter contacts the cam.
As shown in FIG. 9(a) to (c), a nose 53 of a three-dimensional cam 52 changes continuously along its axis. Accordingly, the cam 52 varies the valve characteristics in accordance with the position where the valve lifter contacts the cam.
Generally, a camshaft is manufactured as an assembled unit. In other words, the cam generally described above is attached to a shaft, which is generally cylindrical or columnar and is made of steel. It is necessary to accurately control valve open-close motion in synchronization with piston up-down motion in the engine. Accordingly, when a camshaft is manufactured, high precision is required with regard to the cam assembly angle, or angular position of each cam about the axis of the shaft (called cam assembly phase hereafter).
For example, Japanese Unexamined Patent Publication No. 60-9803 describes a method to determine the cam assembly phase with high precision by the use of a hollow pin. In this method, apertures corresponding to each proper assembly phase are formed both on a cam and a shaft. The cam assembly phase is determined by inserting the hollow pin in the apertures.
Also, for example, Japanese Unexamined Patent Publication No. 60-44659 describes a method for determining the cam assembly phase by engaging a key with a keyway. In this method, the shaft has a keyway on its periphery, and the cam has a key on the inner surface of a shaft insertion hole. The engagement of the key with keyway determines the cam assembly phase. However, it is necessary to form apertures and keyways with high precision in either method. As a result, the camshaft manufacturing cost is high.
On the other hand, when using a usual flat nosed cam (a cam having a constant cam-nose radius), a jig having a generally V-shaped recess is used to adjust the cam assembly phase. As shown in FIG. 10(a) and (b), a shaft (not shown), which has been rotated to a certain angular position, is inserted into a hole 56 of a cam 55, with the nose of the cam 55 fixed in the V-shaped recess of the jig 54. Then, the cam 55 is fixed with respect to the shaft member by a coupling method such as shrink fit. In this case, the cam 55 and the jig 54 make line contact with each other, and the cam 55 is securely held by the jig 54. According to this method, the cam assembly phase is determined easily and precisely without machining the cam 55 or the shaft member in any special way.
However, when the method using the jig 54 is applied to manufacturing a three-dimensional camshaft, the following problems arise. As shown in FIG. 11(a), (b), in three-dimensional camshafts, the nose 53 of a cam 52 is inclined with respect to the axis of the camshaft. The edge of the cam 52 thus makes point contact with the jig 54, and the cam 52 is not securely fixed. This also makes it impossible to precisely position the cam 52 on the shaft. Since there is point contact between the edge of the cam 52 and the surface of the jig 54, the jig 54 and the edge of the cam 52 are frequently damaged.
To control the precision of the cam profile, the cam profile shape is measured. However, in three-dimensional camshafts, it is quite difficult to measure the cam profile, and the cam profile is not as precise. This is because the nose surface is inclined with respect to the shaft axis, and the measured cam profile shape varies axially.